1. Field of the Invention
This invention relates to aircraft fuel gaging systems and, more particularly, to systems utilizing individual multiplexed junction probes in separate fuel tanks.
2. Description of the Prior Art
Known aircraft fuel gaging systems of the prior art typically employ a plurality of probes in each of the separate fuel tanks connected in parallel to develop signals indicating the volume of fuel in each tank. These probes comprise capacitors which are connected so that their admittance varies linearly as an analog function of quantity of fuel in the tank. A compensating capacitor in each tank, totally submerged in the fuel, provides another analog indication that varies with the density of the fuel, based on a predetermined calibration relating the compensating capacitor to density variations of fuels within a standard range. These signals are sometimes directed to master indicators in the cockpit panel which include various circuitry to convert the analog signals to digital output which drive cockpit displays. These signals are also sometimes directed to repeater displays in the aircraft refueling panel.
Gaging systems of the type described, while reasonably accurate for fuels within the standard range for which the compensating capacitors are calibrated, are subject to increased errors in fuel quantity measurement when other, non-standard fuels are used. Such fuel gaging systems depend for accuracy on precise control of the type or class of fuel in the tanks. Because of the variation in quality control in the fuels provided at some airports and because of fuels being refined from new sources of crude oil, there is a much greater variation encountered in present fuel density than was previously the case when the described fuel gaging systems were designed. The typical variation leads to a three or four percent error in fuel quantity measurement. The DC-10 airliner, for example, typically carries 250,000 pounds of fuel; thus, maintaining a reasonable reserve requires carrying at least 10,000 pounds of fuel because of possible errors in the reading of the fuel gaging system as described, due to possible deviations from the standard compensating capacitor calibration.
An improved aircraft fuel gaging system is disclosed in U.S. Pat. No. 4,420,976 entitled Multiplexed True Mass Gaging System of Eugene F. Orloff, Martin Horowitz and Charles H. Ritter, co-inventors of the present application. The system disclosed in that patent incorporates a densitometer in each of the individual fuel tanks, in addition to the plurality of probes and the associated compensating capacitor, to develop a signal indicative of the true mass of the fuel in the respective tanks. In addition, the system of that patent incorporates signal processing apparatus which is coupled between the fuel tank probes and the fuel quantity displays for generating display readouts in accordance wtih tank signals. That signal processing system provides duplication of components in dual channels (for redundancy) over most of the signal path and includes a standard electronic module (SEM) mounted near the central connections to the fuel tank probes and containing appropriate signal conversion, processing and multiplexing stages in duplicate, dual data busses extending from the standard electronic module to the cockpit and refueling panels, respectively, and dual-channel signal-receiving and -processing stages at the cockpit panel as well as dual-channel signal-receiving capability at the refueling panel. A multiplexer is coupled to receive signals derived from the probes and compensator in each tank and to apply those signals in multiplexed form to an A/D (analog-to-digital) converter which in turn provides a digitized output to a microprocessor. The microprocessor applies a data stream, corresponding to the digitized signals, to signal busses leading respectively to the cockpit panel and to the refueling panel. Each data bus is a single shielded, twisted pair of conductors. In the display units, each signal path includes a data receiver coupled to the incoming data bus for providing an output to a demultiplexer which supplies demultiplexed signals to display drivers and associated signal combining stages. The reader is referred to that patent for a more detailed description of the disclosed multiplexed true mass gaging system, and the disclosure of that patent is incorporated herein by reference.
In known prior systems which use junction probes or functionally similar devices that connect in parallel to a plurality of individual tank units in respective fuel tanks, the difficulty of fault isolation of tank components with the concomitant drawn-out trouble-shooting procedures is a significant and costly maintenance problem. To accomplish fault isolation of tank components which would substantially reduce the time and cost of maintenance would, in the system of the cited patent, require bringing out individual wires from each tank unit and compensator to the signal processor unit. Such an approach would substantially increase the number of wire terminations and would also increase the weight of the wiring, the wiring support hardware and associated feedthroughs and connectors.
It will be appreciated that each individual component of installed instrumentation systems on an aircraft represents an increment of dead weight which subtracts from the load carrying capability of the aircraft. Thus, each of the individual fuel tank units, of which there are several in each tank, represents added dead weight; this weight factor is increased by the associated wiring, mounting brackets, etc. which are required for the individual probes. In addition, in the described prior art systems, the failure or inaccuracy of reading from a single tank unit affects the accuracy of the fuel quantity reading for the entire tank. The system of the cited patent incorporates particular circuitry for detecting such inaccurate readings and providing a fault indication to the cockpit display.